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Abstract— The effects of the carboxylic and amino groups in their neutral and charged forms on the absorption and emission properties of phenylalanine are analyzed with the help of model compounds containing either substituent. Fluorescence yields of phenylalanine and model compounds were measured as a function of pH. Fluorescence quenching occurs in going from COO‐ to COOH and from NH3+ to NH2. Lowering the ionization potential of the phenyl group by substitution increases the quenching effect. This provides more evidence that an intramolecular charge‐transfer mechanism may be responsible for intersystem‐crossing rate enhancement and fluorescence quenching in phenylalanine and model compounds.

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